1. Field of the Invention
The present invention relates to a zoom optical system for use in an optical unit or a like device, and more particularly to a zoom optical system having a zoom ratio of about two to five times, as well as an image sensing device incorporated with the zoom optical system, and a digital apparatus loaded with the image sensing device.
2. Description of the Related Art
In recent years, digital cameras are rapidly spread, and are widely used not only as means for inputting images into a personal computer, but also as a tool for keeping photos, as well as conventional silver halide cameras. In particular, digital cameras whose zoom ratio is about three times are widely spread in the aspect of a compact size and superior usability. As the digital cameras are widely spread, a demand for a miniaturized zoom lens is increased. Miniaturization in this context embraces miniaturization both in the size of an optical unit in use, and the size of the optical unit in a lens barrel collapsed condition. Also, since the number of pixels of an image sensor is being increased year by year, there is a demand for higher optical performance. Further, there is a demand for high optical performance, as compared with a conventional arrangement, in an apparatus such as a cam recorder primarily used in recording moving images, considering a still image photographing function and a demand for compatibility with high definition devices.
As a zoom lens system capable of satisfying the aforementioned demands, a three-group zoom lens is widely known, wherein a first lens group having a negative optical power, a second lens group having a positive optical power, and a third lens group having a positive optical power are arranged in this order from the object side toward the image side, as disclosed in e.g. US2005/0259334A1 (hereinafter, called as “D1”). The three-group zoom lens has a reduced error sensitivity resulting from decentering, and is feasibly producible, in addition to high optical performance at a relatively small size.
There is also known a four-group zoom lens, wherein a first lens group having a negative optical power, a second lens group having a positive optical power, a third lens group having a negative optical power, and a fourth lens group having a positive optical power are arranged in this order from the object side, as disclosed in e.g. U.S. Pat. No. 7,042,651B2 (hereinafter, called as “D2”). The four-group zoom lens is advantageous in miniaturizing the optical system, without unduly increasing the optical power of the second lens group or the moving amount of the second lens group, by distributing a zooming load to the second lens group and the third lens group while partially overlapping the moving ranges of the second lens group and the third lens group at the time of zooming.
In the zoom optical system disclosed in D1, the zooming load is concentrated to the second lens group. This necessitates securing the moving distance of the second lens group depending on an intended zoom ratio, or increasing the optical power of the second lens group without increasing the moving distance of the second lens group in order to attain the intended zoom ratio. The former arrangement may obstruct miniaturization of the optical system, and the latter arrangement may make it difficult to correct various aberrations resulting from increasing the optical power. In the zoom optical system disclosed in D1, a meniscus lens element is used as a lens element in the first lens group. Accordingly, the thickness of the optical axis direction of the first lens group may be unduly increased, which may obstruct miniaturization of the optical unit in a lens barrel collapsed condition.
The zoom optical system disclosed in D2 has not sufficiently accomplished miniaturization. The zoom optical system in D2 has a great distortion aberration at a wide angle end.